Switched-mode power supplies with several output voltages are preferably realized by means of transformers with several output windings. In such a switched-mode power supply, a first output potential voltage UA.sub.1 is regulated directly via a converter. The other output voltages UA2, . . . ,UAx, which can be picked off at the outputs of the converter, are respectively regulated indirectly by means of readjustment units so as to meet the requirements of the circuit arranged at the output terminals. This type of switched-mode power supply has the advantage that a multiplicity of output voltages can be produced with only one converter system. However, a disadvantage of this circuit arrangement, shown in FIG. 1a, is that the output voltage UAx to be corrected must be greater than the output voltage after a readjustment unit NRx, which output voltage is to be adapted to a load.
For example, this precondition is very difficult to maintain in the case of extreme load situations at the directly regulated circuit of the converter. Given a too-small load, in particular given no-load operation, at the directly regulated output circuit (e.g. UA1), the target potential voltages at the remaining output terminals of the converter cannot be predetermined for subsequently arranged circuits.
It was previously standard that this cited disadvantage in a switched-mode power supply as described above resulted in the following. With a previous load inserted into the directly regulated circuit, the output voltages UAi (i=2,3, . . . ) at the terminals of the converter are prevented from falling below a target potential in the case of a light load. By a corresponding dimensioning of the multi-winding transformers in the converter with an increase in the number of secondary windings, it is provided that the output potential voltages UAi are sufficiently high, even in extreme load cases on the directly regulated circuit.
The first circuit variant cited in the above section has the disadvantage that power converted into a previous load resistance is lost. Given a light loading, the efficiency of the directly regulated circuit is thereby reduced considerably.
The second circuit variant has the disadvantage that an increased output voltage at the output terminals of the converter is always adjacent to the readjustment units, i.e. even when the directly regulated circuit is not operated with a light load. A disadvantage of this circuit construction is that an increasing degradation of the efficiency of the converter occurs, above all given a higher loading of the directly regulated circuit.
The circuit variants stated above have previously been used for the optimization of a switched-mode power supply in such a way that the losses in the readjustment units were kept at a minimum over a selected load range.
Despite this optimization, in many cases of application a further disadvantage is that the possible output power of all circuits regulated by readjustment units must thereby be smaller than that of the directly regulated circuit, if the switched-mode power supply is still supposed to achieve an acceptable level of efficiency.
In cases in which several powerful output voltages are required, several converter system must also be used.
The underlying object of the invention is to indicate a simple circuit arrangement in which the above-stated disadvantages can be avoided.
In general terms the present invention is a circuit arrangement with several output rectifier circuits for regulating several output voltages of a converter, of which a first output voltage controls the converter by a first output signal. At least one further output voltage is kept constant by a respective readjustment element. A control unit is provided that is connected to the first output rectifier circuit and that regulates the first output voltage. In the control unit a first module is provided for the determination of a first voltage potential at a measurement shunt arranged in series with the control element, and a second module is provided for the determination of a second voltage potential at the switching element. The output of the first module is connected with a first input and the output of the second module is connected with a second input of a third module for the difference value formation. The output of the third module is connected with the control input of the control element.
Advantageous developments of the present invention are as follows.
The first and second modules form a subtraction circuit.
The third module is a PDI controller.
The third module is a P-controller.
The control element is an MOSFET transistor.
A low-pass filter is arranged between the output of the first module and the first input of the third module.
A first capacitor is arranged between an input terminal pair of the control unit, and a second capacitor is arranged between the output of the control element and a terminal of an output terminal pair of the control unit.
A phase control unit is arranged in the feedback circuit forming the control signal for the converter.
The phase control unit is formed with a subtraction amplifier.
The voltage supply of the phase control element can be picked off directly at the converter.
Alongside the advantage that the output voltages, given extreme regulation conditions at the terminals of the directly regulated output voltage, the specified values of the further output voltages are maintained, the invention has the further advantage that fewer losses are produced than in the described solution strategies, and the efficiency is thereby decisively improved.
The invention has the advantage that the circuit arrangement can be dimensioned in such a way that the output power of the corrected circuits is higher than that of the directly regulated circuit. This has the further advantage that further converter systems within the switched-mode power supply are saved, and that the manufacturing costs thereof are thereby significantly reduced.
A further construction of the invention has the advantage that it is always possible to pick off sufficient voltage potential at the output terminals of the converter, and that the corrected output voltages UAi (1, 2 . . . ,3) are independent of a change of load on the directly regulated circuit.
A further construction of the invention has the advantage that, in particular given no-load operation of the directly regulated output circuit, a low-frequency oscillation of the output voltage at the converter does not occur.